Method for operating an internal combustion engine that is equipped with a three-way catalytic converter

ABSTRACT

The invention relates to a method for operating an internal combustion engine that is equipped with a three-way catalytic converter. According to the inventive method, a lambda value of the air/fuel mixture, with which the internal combustion engine is supplied, is set below and above a set value in a cyclically alternating manner during a forced activation whereby the lambda value in rich phases is less than the set value and in lean phases, is greater than the set value. During the forced activation, the rich phases and the lean phases are matched to one another according to a specified criterion. The invention provides that the amount, by which the lambda value in rich phases is set below the set value, is selected so that it is equal to the amount, by which the lambda value in lean phases is set above the set value. When determining the criterion, an air mass is used that is supplied to the internal combustion engine during the rich and lean phases.

CLAIM FOR PRIORITY

This application is a national stage of PCT/DE03/01407, published in theGerman language on Nov. 20, 2003, which claims the benefit of priorityto German Application No. DE 102 20 337.7, filed on May 7, 2002.

TECHNICAL FIELD OF THE INVENTION

The invention relates to a method for operating an internal combustionengine equipped with a three-way catalytic converter.

BACKGROUND OF THE INVENTION

Conventional methods, such as those disclosed in DE 195 11 548 A1, DE198 01 815 A1 or DE 199 53 601 A1, with the last-mentioned document alsodisclosing evaluation of a catalytic converter as regards ageing byevaluating the air mass value of the combustion air that is sucked in bythe internal combustion engine.

In the case of internal combustion engines, emitted exhaust gases can begiven an aftertreatment in the exhaust gas duct by using a three-waycatalytic converter that oxidizes or reduces harmful substances of theexhaust gas to innocuous compounds. However, it is also known that suchinternal combustion engines equipped with a three-way catalyticconverter for achieving a high degree of efficiency must be suppliedwith an average stoichiometric air/fuel mixture; in such a lambdaregulation, the oxygen contents of the exhaust gas is measured by meansof so-called lambda probes and the air/fuel mixture is regulated to anaverage value close to lambda=1 because three-way catalytic convertersonly function in a tight range around lambda 1 as requested. This rangeis also designated as the catalytic converter window.

In order to increase the degree of efficiency of a three-way catalyticconverter, the air/fuel mixture is designed in such a way that in theforced activation acting on the lambda regulation as anticipatorycontrol around the stoichiometric set value, default values are setalternately with an over-stoichiometric and under-stoichiometric mixturein cycles. Because of the forced activation, the default value for thelambda value in rich phases is lower than the stoichiometric set valueand in lean phases is greater than the set value.

Alternately, supplying oxygen to and extracting oxygen from thethree-way catalytic converter results in suitable oxygen ratios for theoxidation and reduction phases.

Because the reducing or oxidizing effect of a three-way catalyticconverter decreases tremendously in the case of values set below orabove a set value for the stoichiometric mixture, care must be takenthat in the forced activation within the average time, an air/fuelmixture is always used in the catalytic converter window.

Therefore, in the prior art in lean and rich phases of the forcedactivation, a default value deviating by the same amount from thestoichiometric set value is set in each case and the phases are equal inlength. Lambda deviations from the default value possibly determined byinterference are balanced out by a lambda regulator.

SUMMARY OF THE INVENTION

The invention relates to a method for operating an internal combustionengine equipped with a three-way catalytic converter in the case ofwhich a lambda value of the air/fuel mixture, with which the internalcombustion engine is supplied, is set below and above a stoichiometricset value in a cyclically alternating manner during a forced activation,whereby the lambda value in rich phases is less than the stoichiometricset value and in lean phases is according to the stoichiometric setvalue, in the case of which during the forced activation, the richphases and the lean phases are matched to one another according to aspecified criterion.

The invention discloses a method of the above-mentioned type in such away that the forced activation brings about a higher degree ofefficiency of a three-way catalytic converter.

The invention discloses a generic method in that for the criterionaccording to which the phases are matched, the air mass is used that issupplied to the internal combustion engine as combustion air in rich andlean phases.

The invention is based on the knowledge that it is important for theefficiency of a three-way catalytic converter to remove again during therich phase the amount of oxygen stored in a lean phase. Because theamount of oxygen by means of which a three-way catalytic converter isfilled in the lean phase and which is removed in the subsequent richphase depends on the amount of air that is fed into the internalcombustion engine as combustion air, the basic approach according to theinvention directly depends on the actual parameters influencing thefilling and removal process. In addition, influences that have an airmass flow that change during the filling and emptying process, no longerhave an interfering effect because they are taken into considerationwhen determining the criterion. Therefore, the invention replaces thepreviously time-based forced activation in the linear lambda regulationwith an air mass flow-based forced activation and as a result againachieves a high degree of efficiency of the three-way catalyticconverter because the catalytic converter window is set in a more stablemanner.

The invention has the further advantage that in rich and lean phases,deviations from the stoichiometric set value can be selected freely andin particular can differ.

Therefore, if the load or the rotational speed of an internal combustionengine changes, the air mass supplied within a unit of time also changesand therefore also the amount of oxygen fed into or extracted from athree-way catalytic converter within a unit of time. Whereas a purelytime-based forced activation has to correct resulting errors via a guideregulator also to be provided for the lambda regulation, the air massflow-based forced activation automatically takes care of a correspondingbalancing, since the lean or rich phases are shortened or lengthened ina corresponding way. As a result, the method according to the inventionmakes the lambda regulation more precise because an error is not onlyeliminated afterwards, but avoided from the start.

Of importance for the air mass flow-based forced activation is the factthat in lean and rich phases, the same amount of oxygen is fed into orremoved from the catalytic converter. In principle, a set amount canthen be specified for this. Alternatively, this set amount can bemanaged dynamically, i.e. a rich phase or a lean phase are ended if theyare matched to the immediately preceding lean and rich phase accordingto the criterion.

In the case of the air mass flow-based forced activation, the air massis used as a criterion for the oxygen mass relevant to a filling orremoval process of a three-way catalytic converter. In a preferredfurther development, a direct volume for the oxygen mass that is emittedin the lean and rich phases in the exhaust gas by the internalcombustion engine can be used as the criterion. For this purpose, theoxygen load during the lean phase can be calculated as follows bysummation or integration of the air mass flow:

${MO2} = {0.23 \cdot {\int_{i = 0}^{i = {TM}}{{( {1 - \frac{1}{LAM}} ) \cdot M}\overset{.}{\; L}\ {{\mathbb{d}t}.}}}}$

This formula gives the oxygen mass MO2 as a function of the absolutelambda value LAM, the flow of the air mass ML and time TM that it takesa lean phase. If instead of the absolute lambda value LAM, the deviationDLAM from a set value 1 assumed for the catalytic converter window isused, the formula is as follows:

${MO2} = {0.23 \cdot {\int_{i = 0}^{i = {TM}}{{( {1 - \frac{1}{DLAM}} ) \cdot M}\mspace{11mu}\overset{.}{L}\ {{\mathbb{d}t}.}}}}$

Therefore, the deviation is the difference between the default value ofthe forced activation and the stoichiometric set value that is adheredto on average. The above-mentioned relationship also applies to the richphase in which oxygen is extracted, however DLAM is then negative.

As can be seen, the concept according to the invention avoids a furthererror which inherently underlies the purely time-based basic approach:it assumes that the oxygen mass supplied to the lean operating phases isthe same as that removed in the rich phases from the catalyticconverter. However, this is not the case because also in the case ofdeviations from the same amount, DLAM of the fraction of the integralbetween brackets is smaller for lean operating phases than for richoperating phases.

The forced activation according to the invention is not based on thisassumption and instead balances the rich and lean phases—and does thisindependently of the selection for DLAM and of the air mass flow. Theintegrated air mass, the average air mass or also the oxygen masscalculated according to the above-mentioned formula can be, for example,the criterion for the oxygen mass. Here the accuracy requirement and thecosts can be balanced.

A particularly accurate regulation of forced activation and at the sametime relatively low costs can be achieved at the same time if, ascriterion, an integral is used over the air mass supplied during therich or lean phase. In addition, the amount by which the default valuein rich phases is set below the stoichiometric set value is selected sothat it is equal to the amount by which the default value in lean phasesis set above the stoichiometric set value. However, this does not haveto be the case. The integral can be executed easily and automaticallytakes different values in the rich and lean phases into consideration.

When adapting a controller to an internal combustion engine type,different parameters are usually set, i.e. applied. Therefore, theoxygen mass can be set in the case of the air mass flow-based forcedactivation. However, in order to be able to achieve the highest possibleparallelism to previous forced activation systems, it is advantageous toapply a time duration as before. For this application, preference shouldbe given to a further development of the invention in the case of whichin each cycle the rich or the lean phase is executed for a specific timethus determining the air mass, and during the subsequent lean or richphase, the air mass is integrated and the phase ends if the air massesare the same.

Therefore, the time provided together with the previous forcedactivation concepts no longer gives both the duration of the lean phaseand the rich phase, but only defines (indirectly) the oxygen mass thatis relevant to the lean or rich phase. The directly subsequent rich orlean phase is then regulated on the basis of the oxygen mass supplied orextracted in the specified time.

Therefore, a first phase (it can be both a lean and a rich phase) thatis executed for a specific time is defined, and which in terms of value,defines the criterion for the composition of the second subsequent phase(in the same way as the rich or lean phase) via the relevant amount ofoxygen or the air mass.

The parallelism to the values used in conventional forced activationconcepts can be increased further if at the start of a first phase (forexample, a rich phase), the current air mass flow from which theinternal combustion engine receives its combustion air is determined anda time is established for which the first phase has to last for thisperiod in the case of this air mass flow in order to achieve apredetermined oxygen mass. Therefore, in the forced activation, thefirst phase is then carried out precisely for this time and indeedindependently from how the air mass flow changes. However, the air massor the oxygen mass during the first phase is detected. The second phaseis developed in such a way that the same air mass or oxygen mass isobtained.

This embodiment of the method provides an air mass or the oxygen mass asthe target value, but which is made available in the form of a time forthe default value of the first phase whereby the highest possibleparallelism to previous forced activation concepts applies with regardto the application of parameters.

The basic approach according to the invention, as is also expressed inthis further development, makes it possible to precisely match theamounts of oxygen removed or fed into the three-way catalytic converterto each another, i.e. the following equation applies:

${\int_{i = 0}^{i = {TM}}{{( {1 - \frac{1}{DLAM}} ) \cdot {ML}}{\mathbb{d}t}}} = {\int_{i = 0}^{i = {TF}}{{( {1 - \frac{1}{DLAM}} ) \cdot \overset{*}{M}}\mspace{11mu} L\ {\mathbb{d}t}}}$

The basic approach according to the invention makes it possible that auniformity is achieved by the specific composition of the lean phaseduration TM as well as the rich phase duration TF. Then, as has alreadybeen mentioned, it is also taken into account that in lean phases thedifference DLAM between the default value and the stoichiometric averageis positive, but is negative in rich phases in the case of which theexpression between brackets in lean phases is less than in rich phases.Over and above that, the default value can now be selected freely in thelean or rich phases and DLAM in particular need no longer be equal tothe amount for the two phases.

The concept according to the invention can be used to particularadvantage in the case of multi-cylinder internal combustion engines withtwo independent cylinder groups that can be supplied with an air/fuelmixture. In order to prevent the independent lambda-regulated cylindergroups drifting apart, it is worthwhile in the case of the conceptaccording to the invention for there to be a forced synchronizationbetween the two groups, for which reason in a preferred furtherdevelopment of the invention care is taken that on ending each secondphase (lean or rich phase) of a cylinder group, the corresponding phaseof the other cylinder groups also ends automatically or that apredetermined phase shift is adhered to.

Therefore, in the case of a multi-cylinder internal combustion enginewith two independent cylinder groups which can be supplied with anair/fuel mixture, a method is preferred which determines a criterion fora cylinder group and is used by default. Therefore, with regard to theforced activation a cylinder group is operated as a master group and theother one follows as a so-called slave group. Therefore, the default bythe master group can take place in many different ways as has alreadybeen mentioned above. However, it is of considerable importance that atspecific times a forced synchronization takes place. For this, an airmass set value, a set value for the average air mass, a set value forthe oxygen mass, etc. can be specified.

In a further development that is very easy to execute as far as controlis concerned, in which the application for a multi-cylinder internalcombustion engine is linked to a further development that can be appliedby a period of time, there is provision, in the rich or lean phase for acylinder group to be determined as the criterion and used as default.Therefore, a rich phase of a cylinder group is carried outtime-regulated and at the same time the supplied air and oxygen mass isdetected. The rich phase of the other cylinder group is then developedaccording to this air mass or oxygen mass value. Likewise, the leanphases of both cylinder groups; in this case care must be taken that thedeviation from the stoichiometric set value in rich phases is not lessthan in the lean phases.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in greater detail on the basis of theaccompanying drawings. They are as follows:

FIG. 1 time sequences of the lambda change and the air mass in the caseof an air mass-based forced activation.

FIG. 2 a flow diagram for carrying out an air mass-based forcedactivation.

FIG. 3 a further embodiment of a method for the air mass-based forcedactivation in the case of which a time value for the application on aninternal combustion engine type can be set.

FIG. 4 time sequences of the lambda change and the air mass in the caseof an air mass-based forced activation for an internal combustion enginewith two cylinder groups which can be supplied independently with anair/fuel mixture.

DETAILED DESCRIPTION OF THE INVENTION

For an internal combustion engine in the case of which a three-waycatalytic converter is arranged in the exhaust gas duct and runs under alinear lambda regulation, in a forced activation a default value is setaround a stoichiometric lambda set value as the anticipatory control forthe lambda regulation. In this case, a shift of the mixture alternatelyto the lean and the rich is given.

In the lean shift, the three-way catalytic converter, that has oxygenstorage properties, is filled with oxygen whereas it is emptied again inthe rich shift. This filling and emptying process depends on thedifference between the default value and the stoichiometric set value inthe phases, i.e. on the amplitude of the forced activation as well asthe duration of the shift.

The amount of oxygen by means of which the three-way catalytic converteris filled and extracted depends on the amount of air that is fed intothe internal combustion engine during combustion. The oxygen mass fedinto a lean phase takes place according to the following equation:

${MO2} = {0.23 \cdot {\int_{i = 0}^{i = {TM}}{{( {1 - \frac{1}{LAM}} ) \cdot M}\overset{.}{\; L}\ {{\mathbb{d}t}.}}}}$in which case ML represents the air mass and DLAM the lambda change,i.e. the amplitude of the forced activation. This equation is alsodesignated as the oxygen mass integral.

In order to now ensure that the filled or emptied amount of oxygen inlean and rich phases of the forced activation is equal, the integral iscalculated in each case. In this case, the lean phase is executed insuch a way that a specific oxygen mass value MO2 is set. The directlysubsequent rich phase is also developed in such a way that preciselythis oxygen mass value MO2 is achieved.

FIG. 1 shows a lambda curve 1 as a time sequence in which case thelambda change DLAM is plotted over time t. The lambda change DLAM isthen possibly approximated to a quadrilateral function during theoperation of an internal combustion engine so that in the half cycles 3and 4, a constant lambda change DLAM is given in each case.

Therefore, the transitions between the half cycles 3 and 4 correspond toa linear change, the slope of which is selected in such a way that inthis case there is no loss of comfort during the operation of aninternal combustion engine.

The lambda value DLAM in each half cycle 3 and 4 is used to calculatethe oxygen mass by means of the above-mentioned integral. Therefore, thelean phase duration TM is the time between two zero passages of thelambda curve 1. As a result, an oxygen mass curve 2 drawn in on FIG. 1in which the air mass ML is recorded over time t is obtained. As can beseen, the oxygen mass integral curve 2 also runs cyclically and issynchronous to the lambda curve 1. At the end of the lean phase durationTM the oxygen mass integral curve 2 has a local minimum.

The end of a lean phase and thereby the end of a half cycle 3 isdetermined on the basis of the oxygen mass integral curve 2. If thevalue of the oxygen mass integral is lower than a value MO2, a switchingpoint 5 is determined in the case of which the lean phase ends, i.e. thelambda change DLAM that was constant up to now then changes to zero withthe above-mentioned slope and then changes to the opposite value for thelean phase. For the zero passages the lean phase duration TM then endsand the rich phase duration TF then follows. From this zero passage, thevalue of the oxygen mass integral again increases. If it reaches zerothen an additional switching point 6 is achieved for which the end ofthe rich phase duration begins and the lambda change DLAM is again setto the value for the next lean phase with the above-mentioned slope.

As can clearly be seen from the lambda curve 1 in FIG. 1, this conceptresults in the fact that the default value in the forced activation isselected and that there are different durations for the lean and richphases. They are in each case developed until exactly the same value MO2is achieved so that a continuous supply in the average stoichiometricmixture is ensured.

This method for the forced activation is shown diagrammatically in FIG.2 which assumes that a rich phase was used as the start. First of all,in a step S1 the internal combustion engine is operated with a slightlyrich mixture, i.e. the lambda value LAM is lowered; this can be seendiagrammatically in step S1 by a minus sign.

Subsequently, the oxygen mass integral is calculated in a step S2. Thiscan be the above-mentioned integral. However, if the lambda value can bekept constant it need not be taken into consideration and an integral orsum formation via the air mass flow alone is sufficient.

Subsequently, a test is performed in a step S3 to determine whether ornot the achieved sum is above a value MO2. Should this not be the case(“N”-branch) it would be necessary to return to step S2, i.e. the richphase is continued.

However, if the value MO2 is achieved on the other hand (“J”-branch),the default value is now raised in a step S4 which brings about a leanermixture, i.e. a lean lambda value LAM is specified. In step S4 this canbe seen by means of a plus sign.

During the resulting lean phase, the oxygen mass integral is againdetermined on the one hand or the air mass is summed up or integrated.This takes place in a step S5.

Subsequently, step S6 requests whether or not this summation againreached the value MO2. If this is not the case (“N”-branch) the leanphase is continued, i.e. step S5 is once again carried out. However, ifon the other hand the oxygen mass value MO2 is achieved (“J”-branch) itwould be necessary to return to before step S1, i.e. a rich phase onceagain follows.

Therefore, in terms of the concept shown diagrammatically in FIG. 2, thelean phases and the rich phases are matched to a same value MO2 in eachcase. It will be possible to select this value depending on theproperties of the three-way catalytic converter and can particularlyalso be increased or decreased for diagnostic purposes deviating fromnormal operation for the short-term, for example, in order to check thebehavior of the three-way catalytic converter.

FIG. 3 diagrammatically shows an alternative embodiment of the method.In this case, in a step S7 a cycle period T is first of all initialized,i.e. set to zero. Subsequently in a step S8 a rich phase to reduce thelambda value LAM is carried out. In step S9, an oxygen mass integralcalculation or the summation or integration of the air mass then followsin the same way as in step S2.

Next in a step. S10, the cycle time T is raised, i.e. increased by onetime increment. A request in a step S11 checks whether or not thecurrent cycle time t exceeds a threshold value SW. If this is not thecase (“N”-branch) the rich phase is continued, i.e. step S9 iscontinued. If, on the other hand, the cycle duration has exceeded apredetermined threshold value SW2 (“J-branch”), the value of the sum orthe integral is stored in a step S12 via the air mass as an oxygen massvalue MO2. It then serves to regulate the subsequent lean phase.Subsequently, the steps S13, S14 and S15 that conform to the steps S4 toS6 are carried out.

The air mass-based criterion for matching the rich and the lean phasesin the forced activation can also for example be used for internalcombustion engines that have several two cylinder groups—the air/fuelmixture of which can be set independently from one another. This isusually the case for internal combustion engines with several cylindersupports, for example, in the case of V6 or V8 configurations.

FIG. 4 shows lambda curves 1 a and 1 b as well as the oxygen massintegral curves 2 a and 2 b for a forced activation in the case of suchsystems.

There is also provision in this case, at certain times, for forcedsynchronizations between the two cylinder groups to be carried out sothat there is no drifting apart of the two groups with regard to theforced activation. Such a drifting apart would be supported by numericalinaccuracies. The lambda curves 1 a and 1 b shown in FIG. 4 provide aforced synchronization at the end of the lean phase of a bank ofcylinders.

In the case of the forced activation, a bank of cylinders is operated asa so-called master, i.e. it supplies the default values with regard tothe air mass-based balancing criterion to the other bank that runs as aslave. The lambda curve of the master-operated bank is provided with areference symbol 1 a in FIG. 4 and is also drawn in with a thicker lineintensity in the same way as the associated oxygen mass integral curve 2a.

The half cycles 3 a and 4 a of the lean or rich phases of the cylinderbank operated as master correspond to those of FIG. 1 so that thesedescriptions can be referred to concerning this matter.

If a switching point 5 a is reached, the end of the half cycle 3 a isimplemented and a half cycle 4 a follows, the end of which is initiatedin the switching point 6. The half cycles 3 b and 4 b of the cylindergroup operated as slave orientate themselves to the oxygen mass valuesMO2 that were reached default-specifically in the case of switchingpoints 5 a or 6. As can be seen from the oxygen mass integral curve 2 bfor the slave cylinder bank that is operated with a push-pull operationto the master cylinder group in the forced activation, the switchingpoint 5 b is reached in time after the switching point, i.e. the halfcycle 3 b takes longer than the half cycle 3 a. The reason for thisbeing the value of the expression in brackets which depends on theindicator DLAM in the above-mentioned oxygen mass integral, shifts inequal amounts DLAM in rich and lean phases.

Therefore, for this reason the half cycle 4 a is also longer than thehalf cycle 4 b. In the oxygen mass integral curve 2 b it stands out thatduring the half cycle 4 b, there is no integration. This is due to thefact that on reaching the switching point 6 that is defined by theoxygen mass integral curve 2 a for the master cylinder group there is aforced synchronization of the half cycles 4 a and 3 b, so that it isensured that the push-pull operation or the specified phase shiftbetween the forced activation of the master cylinder group and the slavecylinder group is retained. However, for the case that a cylinder groupcan be switched off, the integration should be carried on so that theslave support can then be used as the master bank for the short term.

The additional lambda curve 1 a and 1 b as well as the oxygen massintegral curve 2 a and 2 b clearly shows the influence of the oxygenmass integral on the duration of the rich and lean phases and with thatalso the period of the forced activation. There, the oxygen massintegral curve 2 a and 2 b proceeds with a clearly lower slope, i.e. theinternal combustion engine clearly sucks in a smaller air mass flow thanbefore. Therefore, the half cycles 4 b and 3 a are extended accordingly.

Balancing by means of an air mass-based criterion not only brings aboutthat lean and rich phases in each case are the same under the degree ofefficiency viewpoints, but an optimum oxygen mass that is fed into orextracted from the three-way catalytic converter can also be set.

1. A method for operating an internal combustion engine equipped with athree-way catalytic converter, comprising: setting a lambda value of theair/fuel mixture, with which the internal combustion engine is supplied,below and above a stoichiometric set value in a cyclically alternatingmanner during a forced activation, the lambda value in rich phases beingless than the stoichiometric set value and in lean phases being greaterthan the stoichiometric set values; and matching during the forcedactivation, the rich phases and the lean phases to one another accordingto a specified criterion, wherein based on an air mass flow that issupplied to the internal combustion engine in the rich and lean phase, avolume is determined for an oxygen mass relevant to a filling oremptying process of a three-way catalytic converter and used as acriterion to determine the switching points between the rich and thelean phases, and wherein the amount by which the lambda value in richphases is set below the set value is selected so that it is equal to theamount by which the lambda value in lean phases is set above the setvalue.
 2. The method according to claim 1, wherein as criterion, anintegral is used over the air mass supplied during the rich or leanphase and in each cycle the rich or the lean phase is carried out for aspecific time thus determining the air mass, and during the subsequentlean or rich phase, the air mass is integrated and the phase ends if theair masses are the same.
 3. A method for operating an internalcombustion engine equipped with a three-way catalytic converter,comprising: setting a lambda value of the air/fuel mixture, with whichthe internal combustion engine is supplied, below and above astoichiometric set value in a cyclically alternating manner during aforced activation, the lambda value in rich phases being less than thestoichiometric set value and in lean phases being greater than thestoichiometric set values; and matching during the forced activation,the rich phases and the lean phases to one another according to aspecified criterion, wherein based on an air mass flow that is suppliedto the internal combustion engine in the rich and lean phase, a volumeis determined for an oxygen mass relevant to a filling or emptyingprocess of a three-way catalytic converter and used as a criterion todetermine the switching points between the rich and the lean phases; andwherein in a multi-cylinder internal combustion engine with twoindependent cylinder groups that can be supplied with an air/fuelmixture, the criterion determined for one cylinder group is used asdefault for the other cylinder group.
 4. The method according to claim3, wherein in the rich or lean phase of a cylinder group, the criterionis determined as default.
 5. A method for operating an internalcombustion engine equipped with a three-way catalytic converter,comprising: setting a lambda value of an air/fuel mixture, with which aninternal combustion engine is supplied, below and above a stoichiometricset value in a cyclically alternating manner during a forced activation,the lambda value in rich phases being less than the stoichiometric setvalue and in lean phases being greater than the stoichiometric setvalues; in the lean phases, measuring a first air mass flow anddependent on the first air mass flow, calculating a first parameterindicating an amount of oxygen being stored in a three-way catalyticconverter; in the rich phases, measuring a second air mass flow anddependent on the second air mass flow, calculating a second parameterindicating an amount of oxygen being removed from the three-waycatalytic converter; and using the first parameter and the secondparameter to determine the switching points between the rich phases andthe lean phases and to match the rich phases and the lean phases to eachother.
 6. The method according to claim 5, which further comprisesending a rich phase if an amount of oxygen being removed in the richphase substantially equals an amount of oxygen that was stored in animmediately preceding lean phase.
 7. The method according to claim 5,which further comprises ending a lean phase if an amount of oxygen beingstored in the lean phase substantially equals an amount of oxygen thatwas removed in an immediately preceding rich phase.
 8. The methodaccording to claim 5, wherein the first air mass flow and the second airmass flow are flows that are supplied to the internal combustion engine.9. The method according to claim 5, wherein the first air mass flow andthe second air mass flow are exhaust gas flows that are emitted by theinternal combustion engine.